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8/18/2019 appeldoorn 1987 Assessment of mortality in an offshore population of queen conch, strombus gigas L., in southw…
1/8
ASSESSMENT
OF
MORTALIlY IN AN OFFSHORE POPULATION OF QUEEN
CONCH, STRO US GIG S L., IN SOUTHWEST PUERTO RICO
RICHARD S. APPELDOORN
1
ABSTRACT
A Jolly-Seber multiple tag-recapture experiment was conducted for 2 years on a queen conch popula
tion offshoreofLa Parguera, Puerto
Rico
in order to estimate mortality rates. Over 2,000 individuals
were taggedin 9 sampling periods spaced at 3-month intervalsfrom August1983 to August 1985. The
occurrence of fishing in
half
the intervals allowed estimates to made of both fishing and natural
mortality rates. Fishing mortality averaged 1.14 over the study period. An upper limit of natural
mortality including effects of emigration, was estimated to 1.53. Assuming random diffusion,
emigration was estimated and subtracted from the above yielding a corrected value of natural
mortality of 1.05.
The queen conch, Strombus gigas is one of
the
most valuable fishery resources ofthe Caribbean.
Under heavy fishing pressure, stocks throughout
the region have declined Brownell and Stevely
1981 I and
the
need for management has become
increasingly obvious. Ideally, management deci
sions should be based on a firm understanding
of
queen conch biology, stock dynamics, and rates of
exploitation. Estimates of natural
and
fishing
mortality rates are fundamental to understand
ing
the dynamics ofexploited populations
and are
prerequisites of many stock assessment tech
niques, e.g., yield-per-recruit analysis IBeverton
and Holt 1957 .
All previous reports ofnatural mortality rate in
queen conch populations have been made using
only juveniles IAlcolado 1976; Baisre and Paez
1981; Wood and Olsen 1983; Berg 19761. There
are two
primary
reasons for this: 1 populations
where adult queen conch are not fished have
not
been studied or assessed, thus mortality esti
mates would include losses due to fishing as well
as from natural causes; estimates derived from
length-frequency analysis cannot be made for
adults
because growth
in
length ceases
at
the
onset ofsexual maturity. These past estimates of
natural mortality rate have all been high, rang
ing between 1.0 and 4.0, depending upon
the
age
group studied, except those of Wood and Olsen
1983
who reported values of
0.19
and
0.04
be
tween ages 1 and 2 and ages 2 and 3, respectively.
Thus, there is a dichotomy among reported val-
lDepartment of Marine Sciences, University of Puerto Rico,
Mayag\iez. Puerto
Rico
00709.
Manuscript accepted
July
1987.
FISHERY BULLETIN: VOL. 85. NO 4 1987.
ues. Intuitively,
it
is
difficult to predict whether
high or low values of natural mortality rate
should be expected. A low value of
natural
mor
tality rate, comparable to that ofWood and Olsen,
might
be expected for S.
gigas
because,
in
gen
eral, natural mortality is inversely related
to
size
Ursin
1967; Peterson and Wroblewski 1984;
Blueweiss et al. 1978; Pauly 1980 and other
large mollusks, from temperate areas, have typi
cally low values of natural mortality. However,
natural
mortality should also be significantly re
lated to temperature, both directly and through
growth-rate mediated effects Pauly
1979, 1980 .
Thus, higher values should be expected in tropi
cal species. A casual
literature
review revealed
only one est imate of natural mortality
M
in
tropical mollusks for comparative purposes:
M
=
3.66 for the aplysiid gastropod Dolabella au-
ricularia from the Philippines Pauly and
Calumpong 1984 .
In
the
present study, rate ofmortality was esti
mated for an offshore queen conch population
spanning a wide size range and including both
juveniles and adults. The Jolly-Seber multiple
tag-recapture method was used. Separate esti
mates were made during periods of fishing and
nonfishing, thus allowing partitioning of mortal
ity into its fishing and natural components.
MEmODS
The study area was located
in
southwest Puerto
Rico, 7 km south ofLa Parguera Fig. 1 The area
has
a uniform depth of
17
m and consists of a
broad, patchy sand and macroalgal plain with oc-
797
8/18/2019 appeldoorn 1987 Assessment of mortality in an offshore population of queen conch, strombus gigas L., in southw…
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FISHERY BULLETIN: VOL. 85,
NO.4
N
t
.
...
..
---- ...
-_
/
--------
.....
.......
, '--'-- ......._
........ -_
----,'
....-...
FIGURE I. Location ofthe queen conch studyarea (stippled) offLa Parguera, Puerto Rico. Dashed line is the g ofthe insular shelf.
Shaded areas
are
emergent coral reefs.
casional patch reefs. The total study area is esti
mated to be
0.4 km
2
,
although for the first two
periods sampling was limited to approximately
the eastern 50 of the area.
Sampling occurred quarterly, generally
in
the
latter half
of August, November, February,
and
May, resulting in 9 sampling periods spanning 2
years from August 1983 to August 1985. Sam
pling during each period was conducted in the
following manner. The
area
was surveyed by
scuba divers and all data were collected in situ.
Attempts
were
made to locate a minimum of
200
individuals. The maximum number was variable
and subject to limits on queen conch density, the
time dedicated to sampling (generally 3 weeks),
and the weather during that time. During each
period a two-stage haphazard sampling
plan
was
used. Dive sites were located haphazardly
throughout
the full
range
of study area,
and
the
bottom covered during each dive was a func
tion of direction
and
distance travelled, which
also were determined haphazardly.
All
queen
conchs were tagged, when initially
encountered, and measured for
siphonallength
to
the neares t 1 mm using calipers. In addition,
798
adults, defined by the presence of a flared shell
lip, were measured for lip-thickness in a similar
manner. Tags consisted of4.5 cm strips ofDym0
2
label tape, upon which a unique identification
number
was embossed. They were tied around
the
shell spire with nylon line (Fig.
2
The spines
characteristic of queen conch shells held the tag
firmly in place. Upon subsequent sightings tag
number
was recorded
and
shell dimensions re
measured.
During the
2-yr
study period, casual assess
ments were made of fishing activity in the area.
This was easy to
do
routinely
as
fishermen were
willing to
return tags
from fished shells,
and the
presence ofnewly fished, empty shells
on
the
bot
tom (left behind after
meat
extraction)
was
obvi
ous
after
fishing had occurred. When encoun
tered, tag numbers of these shells were recorded.
Data analysis used the Jolly-Sebermethod. The
theory
and
practical mechanics of the method
are
presented
in
detail by Seber (1982: Section 5.1).
is valid for open populations where the effects of
2Reference to trade names doesnot imply endorsementby the
National Marine Fisheries Service, NOAA.
8/18/2019 appeldoorn 1987 Assessment of mortality in an offshore population of queen conch, strombus gigas L., in southw…
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APPELDOORN: MORTALITY IN QUEEN CONCHS
FIGURE
2 Posterior
view
ofadult Strom
u
gigas
with numbered
tag
tied around
the
shell
spire.
mortality, emigration, and recruitment can be
significant between sampling periods. The as
sumptions ofthe method and
their
relation to the
present sampling design are presented in
the
Dis
cussion.
R SU TS
Over 2,000 individual queen conchs were
tagged, spanning a range from 9 cm to 28 cm
in
length. Adult queen conchs averaged 24 cm in
length and represented between 22 and 59 of
sample pOpulations, varying due
to
fishing activ
i ty and juvenile recruitment.
Tag
return
data are given in Tables 1 and 2
according to the methods
in
Seber 1982 , with
the
results of
the
analysis given
in
Table. 3. At
any
given time, roughly one-quarter of
the
popu
lation in the study area was tagged Table 3, col
2 , Note the effect ofincreasing
the
sampling area
at time 3 had on the estimate ofabundance N at
that time and on the estimates ofsurvival
8
and
TABLE 1 . -
Tabulation of the number of queen conchs caught in the i h sample last caught in the
hth
sample after Seber 1982). i and h designate sampling periods;
n;
= number of conchs
caught
in
the ith sample;
R;
= number
of
conchs released at the ith sample;
h
= total number
of
conchs recaptured. last caught
in
the hth sample; m; = number
of
conchs recaptured
in
the ith
sample.
i 1 2 3 4 5 6 7 8 9
n;
424
196 275 228
5
375 262
3 227 Total
R;
424
196 275 228
352
375 262
3
h)
h
1 42 28 19 22
8
1
0
0 120
2 18 7
17
6 0
0 0 48
3
10 9
2 0
2
0 23
4 40
14 5
1
1
6
5 76 22 3 2 103
6 37
13
4
54
7 35
22
57
8 36 36
m; 0
42 46 36 88 106
65
54 65
799
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FISHERY BULLETIN: VOL. 85.
NO
TABLE
2
Tabulation of the number 1queen conchs caught in the ith sample last caught in or
before the hth sample alter seber 1982). i and
designate sampling periods.
Z j
= number 1
different individuals caught before the ith sample which were notcaught inthe th sample, butwere
caught subsequently, and is equal to the sum across the
row
excluding the diagonal element.
2 3
4
5 6
7
8 9 Total
1
42 28
l i
22
8
1
0 0
z2=7e
2
46
26
39 14
1
0 0
z3=80
3 36 48 16 1 2 0
z4=67
4 88 30
6 3 1
zs=4O
5
106 28 6 3
Ze=37
6
65
19 7
z7= 26
7 54 29
zs=29
8
65
TABLE
3.-Population
estimates for queen conch derived from the Jolly-Seber multiple mark-recapture analysis.
MIN
= proportion
of
marked conchs in the population just before sample i; N = population abundance just before sample iwith standard deviation aININ and
coefficient of variation ir N S = proportion sUfviving from the ith to i + 11thsample with standard deviation irIS) and coefficient of varialion
alS; B
= number 1new individuals joining the population between the ith and i
+
1) th sample with standard deviation
alB]
and coefficient
1variation
alB
MIN
N
aININ] alN
S
alS]
alS
B
alB]
alB
1
0.8387 0.1196 0.1426
2
0.214
1,629
310
0.1905 1.8956
0.4474 0.2360 2,585 1,017
0.393
3
0.167
5,673
1,437
0.2532 0.2372
0.0535 0.2257
409 324 0.792
4
0.158
1,754
341 0.1946 0.4706 0.0626 0.1330
62
148 2.398
5
0.250
888
106 0.1191
0.7359
0.1096 0.1490
608 136 0.223
6
0.283
1 261
196
0.1557 0.2913
0.0493 0.1692
361 89 0.246
7 0.248
729
117 0.1602 0.6208
0.1202 0.1937
542 152 0.280
8
0.234
995
205 0.2059
9
0.286
recruitment B between times 2
and
3. These
figures
are
obviously unrealistic e.g., survival
cannot
be
greater
than
one) and
are
not consid
ered in any subsequent analysis or discussion.
Survival rates ofqueen conch varied from 0.839
to 0.237 with coefficients of variation varying
from 13.3 to 22.5 . Intensive fishing was
known
to
have occurred in the study area be
tween samples 3
and
4, and 6 and 7 Fishing also
occurred between samples 7 and 8, but to a much
lesserdegree. In the former two periods estimated
survival rates were the lowest found: 0.237
and
0.293, respectively. Survival estimates from all
other periods represent
the
effects ofonly natural
mortality
and
emigration. The average ofthese 3
values is 0.6817. This would correspond to
an
in
stantaneous
rate
of natural mortality per 3-mo
period of 0.3832
[=
-In
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APPELDOORN: MORTALITY IN QUEEN CONCHS
ability equal for all individuals.
that
rates ofsur
vival are equal for all individuals,
and that tag
loss is negligible. ofield study can hope
to
ma
nipulate the environment
to
such a degree
that
all assumptions are perfectlymet. Thus, potential
deviations from these assumptions
must
be con
sidered and the ir significance and resulting im
plications assessed Begon 1983 . Presented
below is a review of each assumption. Based on
known or suspected aspects of the biology and
ecology of queen conch, potential problems with
respect
to
each assumption are raised. Each po-
tential problem is then considered in light of the
specifics of
the
present study, and an attempt is
made
to
assess its significance, if any. Finally,
consideration is given
to
the robustness of Jolly
Seber estimates.
n
general, it will be shown
that
deviations from Jolly-Seber assumptions,·
if
present, would be small and have
little
impact,
and in
addition,
the
Jolly-Seber method is robust
to such deviations.
n the presentstudy, sampling was conducted
on an
areal
basis. Although sampling was not
truly random i.e., with areas predetermined in a
random fashion . dive sites were spread through
outthe study area and chosen without prior refer
ence to, or knowledge
of
the specific
nature
of the
habitat or conch density
in
the immediate area.
The two stage sampling design employed allowed
every section of the study area
to
have an equal
probability of being covered. The assumption of
equal catchability is
the
more important property
here. and
there
were several points where poten
tial departures from this assumption could have
arisen. Queen conchs usually showed a distinct
clustered distribution. Within a cluster all indi
viduals were felt
to
have an equal probability of
capture, regardless of size. However, large queen
conchswere easier
to
see at a distance. Thiswould
affect capture probabilities
if
distinct size groups
characterized clusters; thus , clusters of large
queen conchs would be noticed more frequently.
While
it
was thought
that
there existed areas dif
ferentially characterized by
the
abundance ofju
veniles and adults, these areas generally occurred
on a
larger
scale than typically covered on a dive.
The random allocation ofdive sites, then, should
have minimized any effect of this heterogeneity.
The tendency of very small queen conchs
to
re
main buried is a second factor potentially affect
ing catchability. Thus, a small tagged queen
conch
«
em might have a reduced probability
of recapture until
it
grows large enough
to
be
exposed most of the time. However, only a very
small fraction of queen conchs sampled would
have been affected by this. By
the
time
the
ani
mals are large enough to be sampled
in
signifi
cant
numbers, they will, by the time ofnext sam
pling, have grown
to
a size where bur ia l is not a
problem;
thus
recapture would not be affected.
n
sum, for the size range sampled,
it
is thought that
departures from
the
assumption of equal catch
ability are small, if present. Pollock and Mann
1983 and Carothers 1973 have both found
Jolly-Seber estimates of survival to be robust
against
heterogeneity of capture probabilities.
Mortality of queen conch could have been de
pendent uponmark status
if
tags attracted preda
tors. There is
no
direct evidence e.g., observa
tions or signs of predators or predation during
tagging periods
to
indicate
that this
problem
oc-
curred, implying that it did not. Tags quickly
fouled with macroalgae and were subsequently
impossible for divers to see
at
any
distance. Thus,
any tag attraction effect could only occur immedi
ately after initial tagging. As such, the problem is
thought to be negligible. Departures from equal
probability of survival may have occurred owing
to
size related processes,
bu t
not
to
a serious de
gree. aturalmortality is thought
to
decrease
with age, with
the
effect being predominant
among small juveniles Appeldoorn in press a, b .
However, the majority of queen conchs sampled
here were large juveniles and adults, which
should have had similar survival probabilities.
Although fishermen usually
take
all sizes of
queen conchs found, large conchs should have suf
fered a greater mortality because generally fish
ermen will not dive
in an area
unless conchs are
visible i.e., large from the surface or on shallow
test
dives. Thus, areas predominantly character
ized by small queen conchs should have been
undersampled by fishermen. Again, the potential
effect of this is offset because
the
majority of
conchs were large. Also, only a portion of the
small queen conchs were from areas particularly
characterized by such individuals. Lastly, rate of
dispersion is also a function of size Hesse 1979;
Appeldoom and Ballantine 1983 , so the emigra
tion component of survival could have been
higher for larger individuals. This,
to
some de
gree, would counter
the
size-mortality trend, with
the effect being predominant among small juve
niles. As such,
the
effect
here
should also have
been minor. nsum, there is no reason to suspect
substantial departures from the assumption of
equal probability of survival, and if survival is
independent of mark
status
and probability of
801
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capture is
independent
of age, Jolly-Seber sur
vival est imates are not
greatly
affected by age
dependent
mortality
(Seber 1982).
There
are methods available for determining
age-dependent mortality rates from Jolly-Seber
data (Pollock 1981; Pollock and
Mann
1983),
Queen conchs, however, aFe difficult to age . Ap
peldoorn 1987), Length groups could be used,
but
these
would
have
to be defined narrowly
so that
all individuals could grow into
the
next largest
size
group within the
3 months between sampling
periods, thereby reducing sample sizes within
length
groups to impractical levels. Data pooled
over
time
would
have
forced confounding
ofnatu-
ral and fishing
mortality
effects, which
was
deemed undesirable.
Tags were held securely in place because spines
on
the
queen conch shell spire prevented
tie
lines
from slipping off,
and
they became more secure
over
time
due
to
fouling. Numbers
remained
readable throughout the 2-yr period. In only those
individuals with poor spine development could
the tag be lost, and then only within the short
time prior to fouling. These included some very
small
juveniles «
cm)
and
a few very old queen
conchs
where
spines
had
been eroded. The
total
number
of such potential cases
was
exceedingly
small
and
is, therefore considered negligible.
stim tion
of
migr tion
n
atural
ortality
Permanent
emigrat ion of queen conch
was
known to occur. Reports from fishermen placed
some individuals as much
as
9 km away from the
study area although the time this took is un
known. The degree
of
emigration needs to be ac
counted for
if
a more accurate estimate of natural
mortality is to be obtained. Emigration can occur
by either of two dispersion processes: random dif
fusion,
and
directed migration
or
drift. Seasonal
migrations are
expected in
late
fall
and ear ly
spring
based
on other studies (Hesse 1979; Appel
doom 1985),
although the
expected distance
trav-
elled is unknown. However, no evidence of such
migration
is apparent in
the data
for either mor
tality or
recruitment.
Possible reasons for this
are
1 the
mortality component
is
confounded by fish
ing
effects
thereby masking
seasonal trends, 2
Jolly-Seber
estimates
of are typically imprecise
(Seber 1982), and 3) sampling periods were too
few and inopportunely placed. Since the data do
not support the occurrence of significant drift, at
least to th e point where it can be partitioned from
802
FISHERY BULLETIN: VOL. 85.
NO.4
diffusion,
the
degree
of
emigration was
estimated
by analyzing diffusion only.
Skellam
(1951) presented a two-dimensional
diffusion model which
can
predict the proportion
ofa population
P,
outside the area
of
radius
(PI)
in
a given
time
t if the average distance trav
elled (E) peF unit time M is know-no Assuming no
mortality or
birth
the
equation is
To
estimate
emigration
an
average
value
of E is
needed. Dispersal ability in
trombus is related
to size (Hesse 1979; Miller 1972; Appeldoorn
and
Ballantine
1983). Specifically, if
total
movement
is expressed solely as diffusion,
the
data
of
Hesse
(1979) from a 1.5-mo period encompassing
episodes ofboth diffusion and ri t indicated that
adults travelled twice as fast
as matu ring
queen conchs (adults
with
thin
lips
or very large
juveniles)
and
three times as fast as juveniles.
Hesse (1979) recorded adults to move commonly
50-100 mlday, bu t no average figure was given,
and it is assumed that rates less than this were
also common. Clifton
et
a ' (1970)
tracked
one
group of
adult
queen conchs
at
45-55 mlday over
several days. Given
these rates
a
value of
50
mI
day
seems a reasonable
estimate of E
for adults,
averaging
higher values
during
migration with
lower values
at
other times.
The La
Parguera
population consists
of both
juveniles and adults,
so
this
value needs to be adjusted downward ac
cordingly. Since the majori ty of
queen
conchs
were old juveniles
or
young adults, an average
stage of
maturing can
be assumeed, and a
value
of E
one half that for adults would be most appro
priate, Le., E 25 mlday. it is assumed that the
0.4 km
study area is a circle
of
radius ca. 350 m,
then in one samplingperiod (90 days) 11.2
of
the
population would
be
expected
to
emigrate.
This
results
in
an instantaneous
rate of
annual
emi
gration equal to
0.481.
Subtracting
this from
the
estimate
of
natural mortality
plus emigration
(1.533) yields a corrected
estimate
of
natural
mor
tality
rate
at
1.05.
This value
of
natural
mortality
rate is lower,
bu t consistent
with values
reported
by
Alcolado
(1976), Berg (1976),
and
Baisre and
Paez
(1981),
which might
be
expected since their
estimates
were l imited to juveniles. However, it is still
much greater than those reported by Wood and
Olsen (1983). Hoenig (1983) presented empirical
equations predicting
mortality
rate on the basis
of oldest known age, which can be used for com-
8/18/2019 appeldoorn 1987 Assessment of mortality in an offshore population of queen conch, strombus gigas L., in southw…
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APPELDOORN: MORTALITY IN QUEEN CONCHS
parative purposes. Given a maximum life span of
7 years Wefer
and
Killingley 1980)
the
predicted
mortality rate is 0.72, a r es ul t compatible w it h
the
above
estimate
of
natural
mortality
rate
con
sidering
the
var ia bi lit y associated w ith each
estimate.
tatus
o
th
ishery
The value of fishing mortality
F
relative
to
natural
mortality M indicates that
the La
Par
guera S. gigas population is intensely exploited.
Gulland s 1971) calculationofF = at
the
point
ofmaximum sustainable yield
MSY)
would indi
cate, allowing for variabili ty
in the
estimates,
that the population was being fished at or above
s
at
this
time. However, Francis 1974) has
shown
that
this
re lati onship does not alw ays
hold,
and
Caddy
and
Csirke 1983) stated
that
tropical species
in
particular, already character
ized by high levels of
natural
mortality, would be
more likely
to
be overfished according to
this
for
mula. Using
this
conservative approach, a diag
nosis of overfishing seems warranted. Since
the
study
area
is representative of that portion of
the
offshore
La
Parguera
s he lf s upporting queen
conch fishing,
in
terms of habitat, conch density,
and general fishing activity, these results should
have general relevance.
KNOWLEDGMENTS
M any people joined me for long hour s under
water; particular
thanks go to D.
L. Ballantine,
A. T.
Bardales,
J
Colley,
G.
Gonzalez,
J J
Kim
mel, I
M.
Sanders,
and Z.
Torrez. Suggestions
from
D.
L. Ballantine
and
three anonymous re
viewers improved
the
manuscript.
G.
Collazo
drew
the
figures. This work was supported
in
part
by gra nt s from
the
U.S. National Marine Fish
eries Service NA81-GA-C-00015).
LITER TURE
ITED
ALCOLADO. P.
M.
1976. Growth, morphological variations of
the
shell
an d
some biological dataofthe conch Strombu8 gigas L. Mol
lu sca, Mesogastropoda). Acad. Cienc.
Cub
Ser.
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Recommended